Newborn rats acquire immunity passively by receptor-mediated uptake of maternal immunoglobulin G (IgG) from the first milk. Specific IgG binding to brush borders and IgG transport across the gut increase concomitantly for 10-12 days after birth and then fall until closure at about 21 days. Cortisol acetate administration accelerates this decline. Two classes of binding site are resolved by their affinities (K A1= 1.3 X 108M; K A2= 5.15 X 106M). Persistence of the low-affinity site after closure precludes a transport role (see Rodewald et al, this volume). Target size analysis gives a preliminary M, for the high-affinity site of 90 000-100 000. IgG recognition involves a small number of positively charged residues in the Fc region. An Fc binding activity is solubilized from intestinal brush borders by lithium 3, 5-diiodosalicylate.